Heteroepitaxial semiconductor structures can be found in many semiconductor devices, including electronic devices such as bipolar transistors, and opto-electronic devices such as LEDs and laser diodes. Although such devices generally are embodied in III-V semiconductor material, this is not necessarily so, and Si/Si.sub.x Ge.sub.1-x heteroepitaxial structures are known. Furthermore, it is likely that II-VI heteroepitaxial semiconductor devices will be commercialized in the future. Many devices (e.g., various laser types)comprise periodic heteroepitaxial structures.
By a "periodic" heteroepitaxial structure we mean a structure that comprises two or more pairs of semiconductor layers of predetermined thickness. Typically, the layers in a periodic heteroepitaxial structure are relatively thin, exemplarily less than 250 nm or even 150 nm.
By definition, a given pair of layers in a periodic heteroepitaxial structure comprises a first single crystal semiconductor region of a first composition that is contactingly overlain by a second single crystal semiconductor region of a second composition that differs from the first composition, with the crystal structure of the structure being continuous across the interface between the first and second regions. Generally, the lattice constant of the first composition material differs from that of the second composition material by at most a few (typically {2) percent.
A conventional technique for growing periodic heteroepitaxial structures is molecular beam epitaxy (MBE), which involves exposing an appropriate semiconductor substrate to a flux of particles (atoms, small clusters of atoms, or molecules) from one or more effusion cells. The substrate conventionally is maintained during MBE growth at a constant elevated temperature, and the composition of the incident flux is changed by means of a shutter or shutters, with all temperatures maintained constant. Recently, shutterless MBE growth, with the flux composition changed by changes in cell temperatures, has been disclosed. See M. Hong et al., Journal of Crystal Growth, Vol. 111, p. 1071 (1991); and M. Hong et al., Journal of Electronic Materials, Vol. 21, p. 181 (1992).
Other techniques for growing heteroepitaxial structures, including periodic ones, are also known. These include chemical vapor deposition (CVD), and variants thereof (e.g., MOCVD). All of these growth techniques have in common that typically the substrate is maintained at a fixed, predetermined elevated temperature, with composition changes effected by change of the composition of the growth medium, e.g., the precursor gas in contact with the substrate. By "growth medium" we mean herein the medium that provides the constituents of the growing semiconductor material. The term thus includes the particle flux of MBE as well as the precursor gas of CVD. It does not include, however, a melt.
As those skilled in the art will readily appreciate, in device applications the quality of the semiconductor material of the periodic heteroepitaxial structure, including the quality of the interface between two regions that differ in composition, is of major concern. Furthermore, high precision in layer thickness and composition is required in many devices that comprise periodic heteroepitaxial structures, e.g., distributed Bragg reflector mirrors in laser diodes. However, it is frequently at best difficult to achieve, by conventional growth techniques, the required precision of a few percent in layer thickness and composition of periodic multilayer structures, and at the same time obtain the required high quality in both the first and second composition materials.
Thus, a new growth technique which makes possible, inter alia, growth of highly precise periodic heteroepitaxial structures that consist of semiconductor material of high quality, would be highly desirable. Such a technique could yield devices of improved performance. This application discloses such a technique.